After you use GNU Privacy Guard, or really any public-key encryption system, for a while, you'll probably have it set up for more than one of your email addresses. It's tempting to have the same pass-phrase for all your IDs.
For instance, I use long-ish pass-phrases that are similar enough to remember easily but different enough that a dictionary attack is highly unlikely to be successful. That also protects me from doing something silly/confusing/potentially dangerous like thinking I'm sending from one email address when my email package actually defaults to another. Since the pass-phrases differ, you can't sign a message sent with Account B with the phrase from Account A (that you thought you were using but you were in too much of a hurry to pay attention to the 'From' line).